Please help??

I had a question for you experts. I have tall Fescue that I planted by seed. At the time of planting I was just throwing out grass seed to to keep the weeds from growing. It was our first home and we couldn't afford to landscape. To make a long story short the lawn came in very well. It came in so well I later added a sprinkler system.... I know I went about it backwards. The lawn has been in for about two years and looks great for the most part. My question is. Is there something I can put on the lawn to help loosen up the soil? I have a few spots that seem to turn brown no matter how much I water. I'm sure it's rocky clay soil that is causing the problem. Would something like gypsum work or help. She I apply it to the whole lawn? I like to be the envy of the neighbors and have the nicest, greenist lawn. Please help me out with any suggestions.

BTW- I use a Honda Harmony II mower. (this years model) I've pretty much been mulching once a week. I keep the lawn about 3 to 3 1/2" tall. I use Scotts turf builder about every 3-5 months. Please take the time to reply with any advice. Oh, I almost forgot.. I live in Sacramento, CA. It was 106 two days ago. Were going to have a few days in the 80's.

I'm not sure if I've diagnosed the prblem cprrectly. I do know the grass in the bad area's are NOT dead. I've been really watering those area's the last three day and it's look soooooo much better. I would like to improve the soil in those areas.